1. Field of the Invention
This invention relates to the production of dehydroacetic acid and more particularily to a process of purifying dehydroacetic acid.
2. Prior Art
Several processes are known whereby dehydroacetic acid is produced from diketene. Various catalysts are available for that reaction and that reaction can be carried out in various different solvents. The dehydroacetic acid obtained in those various prior art methods, particularly if the solvent is used several times, shows very considerable discoloration. As dehydroacetic acid can be used as a food preservative, any impurities in it are undesirable, particularily colored impurities which would be visible to the consumer. Impurities are also undesirable since one of the uses of the dehydroacetic acid is in foods.
Swiss Pat. No. 362,700 describes processes for the production of dehydroacetic acid by the dimerization of diketene in an inert solvent at a temperature below 30.degree.C., in the presence of a tertiary amine (as the catalyst) and in the absence of water. Dehydroacetic acid is crystallized; filtered out of the reaction mixture; washed with toluene; dried; and then purified by distillation after being mixed with methyl acetoacetate (specifically, see page 2, lines 11 to 15 and 42 to 54). This reference does not disclose or suggest the purification of dehydroacetic acid by means of reduced pressure or simple vacuum distillation in the presence of activated charcoal or carbon. Methyl acetoacetate acts as a co-distillant in the distillation step of this reference. The use of that, or any co-distillant, results in very serious disadvantages in the distillation purification process. An expensive distillation step and or subsequent separation scheme are needed when a co-distillant is used.
French Pat. No. 1,040,626 discloses a process for the purification of dehydroacetic acid by co-distillation of a mixture, which contains impure dehydroacetic acid and a glycol compound, at a distillation temperature of 60.degree.C. at 1 mm pressure to 150.degree.C. at 15 mm pressure. The process then involves crystallization to obtain the dehydroacetic acid. This reference does not disclose or suggest the purification of dehydroacetic acid by means of reduced pressure or simple vacuum distillation in the presence of activated charcoal or carbon. The glycol compound acts as a co-distillant in the distillation step of this reference. The use of that co-distillant results in very serious disadvantages in the distillation purification. An expensive distillation step and a subsequent separation scheme are needed when a co-distillant is used.
French Pat. No. 1,121,186 teaches a process for the production of dehydroacetic acid by the dimerization of diketenes. The crude dehydroacetic acid, resulting from the dimerization, is purified by distillation at an pressure of 10 to 25 mm below atmospheric pressure and using a distillation temperature between 200.degree. and 270.degree.C. A mixture of hydrocarbons is first added to the crude dehydroacetic acid. The hydrocarbon mixture acts as a co-distillant in the distillation step of this reference. The use of that co-distillant results in very serious disadvantages in the distillation purification process. An expensive distillation step and a subsequent separation scheme are needed when a codistillant is used. This reference does not disclose or suggest the purification of dehydroacetic acid by means of reduced pressure or simple vacuum distillation in the presence of activated charcoal or carbon.
Swiss Pat. No. 401,086 discloses a process for the purification of dehydroacetic acid by distillation of melted dehydroacetic acid in the presence of an inert gas at a temperature of 100.degree. to 200.degree.C. An aromatic hydrocarbon is also present and acts as a co-distillant. Then, after liquefying the gas by the cooling thereof, dehydroacetic acid is separated out by crystallization. This reference does not disclose or suggest the purification of dehydroacetic acid by means of reduced pressure or simple vacuum distillation in the presence of activated charcoal or carbon. The use of the co-distillant in this reference results in very serious disadvantages in the distillation purification process. An expensive distillation step and a subsequent separation scheme are needed when a co-distillant is used.
Any purification process which uses a co-distillant is expensive because it requires additional processing steps after distillation to remove or separate out the co-distillant. Further, there is the added expensive of having to distill the large volume of co-distillant. Also, reduced pressure or simple vacuum distillation cannot be used when a co-distillant is admixed with the crude dehydroacetic acid. In one sense, the introduction of a co-distillant is merely the addition of large amounts of an impurity which must be subsequently removed. Examples of known co-distillants which have been used in distillation separation schemes for difficult-to-remove impurities from crude dehydroacetic acid, are methyl acetoacetate, aliphatic hydrocarbons, aromatic hydrocarbons and various glycols.
The co-distillants are additives which come off with the dehydroacetic acid and subsequently can be separated from the dehydroacetic acid. There is no chemical conversion of the codistillate. The known co-distillates all appear to be liquids; for example, methyl acetoacetic acid is liquid at room temperatures and glycol (ethylene glycol) is a liquid at room temperature.
Attention is also drawn to U.S. Pat. Nos. 2,125,383, 2,229,204, 2,729,652, 2,849,456, 2,997,482, 3,367,847, 3,408,267, and 3,575,816, plus: Brown, "Unit Operations", Weley & Sons, Inc. (1951), pp 398 and 399; Lauer, "Chemical Engineering Techniques," Reinhold Pub. Co., (1952), pp 412 and 413; Perry et al., Chemical Engineers Handbook, 4th Ed., MccGraw-Hill, (1963), pp 16-2 to 16-5; "Rompps Chemie Lexikon," 7th Ed., Vol. 2, p. 792; "Ullmanns Enzyklopadie der Technischen Chemie," 3rd Ed., (1961), Vol. 2/1.pp. 53 and 54.